1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as an image heat fixing device in an image forming apparatus such as a copying apparatus or a printer, and more particularly to an image heating apparatus having a flexible sleeve.
2. Related Background Art
Recently, as a heat fixing device to be mounted on a copying apparatus or a printer, there is desired a heat fixing device with a short start-up time to a fixing temperature and with a low total electric power consumption during the printing operation and the waiting state. In order to meet such requests, there is commercialized a heat fixing device utilizing a flexible sleeve (also called a fixing film, a fixing belt or a film) as a sleeve coming into contact with a toner image. Heat fixing devices of film heating type are disclosed for example in Japanese Patent Application Laid-Open Nos. S63-313182, 2003-045615, 2003-156954, H10-10893 and H11-15303. The flexible sleeve employed in the heat fixing device of film heating method may be based on a heat resistant resin such as polyimide or on a metal such as stainless steel having a higher thermal conductivity than the resinous material.
In such heat fixing device of film heating type, a film rotation induces a phenomenon that the film is moved (or biased) in a thrust direction (direction of generatrix), and it is difficult to produce the apparatus free from such movement. Thus, Japanese Patent Application Laid-Open Nos. H04-44075 and H04-204980 disclose a structure of maintaining the film free from a tension as far as possible, thereby suppressing a biasing force of moving the film in the thrust direction. In the apparatus described in these literatures, the film is not driven by a tension roller or a driving roller provided inside the film but by a pressure roller (pressure roller driving method). As it is still difficult to completely eliminate the film movement in the thrust direction even in the apparatus of such pressure roller driving method, flanges are provided in positions corresponding to edge positions of the film in order to restrict the film movement by such flanges.
However, in a configuration of restricting the film movement by the flanges, in case a biasing force of the film in the thrust direction becomes larger than a designed value for example by an influence of tolerance in the components, a large load is applied to the edge portion of the film thereby deteriorating the durability of the film. In particular, as the film is nipped in a nip portion, the film shows a low flexibility in an upstream region and a downstream region of the nip portion in the rotating direction of the film, and the film tends to show a breakage when edge portions of the film in such regions are restricted by the flanges.
In order to avoid such drawbacks, Japanese Patent Application Laid-Open No. H05-208750 proposes a shape of a film restricting face of the flange. FIG. 19 schematically shows an apparatus described in this patent literature. Referring to FIG. 19, there are shown a film 203, a pressure roller 204, a pressure roller driving gear G meshing with an unillustrated motor gear, a flange 202, a film interior inserted portion 202a of the flange, a flange face 202b, and a pressure spring 205 for forming a nip portion N. The nip portion N is formed between a heater (not shown) inserted into the interior of the film and the pressure roller 204 under the force of the spring 205. The film 203 is nipped, in the nip portion N, between the heater and the pressure roller 204. The film 203 is rotated by a rotation of the pressure roller 204.
As shown in FIG. 19, the flange face 202b has a tapered shape at the nip side. Such tapered shape serves not to restrict the edge portion of the film at the upstream and downstream sides of the nip portion, thereby suppressing the deterioration of the durability of the film.
However, a breakage in the edge portion of the fixing film may still occur even when the flange is formed in a tapered shape. FIG. 20 schematically shows a state in which the entire flange 202, for restricting the rotating trajectory of the fixing film 203 and the edge portion thereof in the thrust direction, is somewhat inclined to an outward direction from the thrust direction for example by an assembling tolerance of the flange 202, wherein the fixing film has a biasing force and impinges on the restricting face. In a state shown in FIG. 20, since an outward inclination of the entire flange 202 causes an inclination of the restricting face 202b also to the outward direction, the edge portion of the fixing film impinges, by the biasing force, on the restricting face in a localized area closer to the fixing nip portion N (an area indicated by a broken line). It is clarified by the investigation of the present inventors that a breakage is generated in an edge portion of the fixing film in case a strong biasing force is exerted on the fixing film in this state. It is also clarified that, in case the flange 202 is inclined in an outward direction, a breakage is generated by the biasing force also in an upstream or downstream vicinity of the fixing nip portion N by an impingement of the edge portion of the fixing film.
This is principally because the flexibility of the fixing film itself tends to be lost in a vicinity area of the fixing nip portion N in which the fixing film is nipped and a localized impingement of an edge portion of the fixing film on the restricting face of the flange 202 causes a strong stress to such edge portion, and a film breakage is generated in case such stress exceeds a breaking strength of the fixing film.
Particularly in a film (fixing belt) having a metal layer as a base, the fixing belt itself has a high rigidity but, in a process for producing such fixing belt, burrs and small cracks are generated on edge faces thereof in cutting the fixing film into a length suitable for a fixing apparatus and fissures are generated from such burrs and small cracks by a frictional contact in the course of repeated rotations.
In order to prevent such fissures on the edge portions of the fixing belt, Japanese Patent Application Laid-Open No. 2002-231419 discloses a configuration in which a flange, supporting an edge portion of the fixing belt, is supported from an external periphery side of the fixing belt and is made to rotate together with the fixing belt, thereby avoiding a frictional contact between the edge portion of the fixing belt and the fixing apparatus.
FIGS. 21 and 22 are respectively a perspective view and a cross-sectional view, seen from a longitudinal direction, of a configuration in which a fixing flange 300 is supported from the external peripheral side of a fixing belt 301 and is rotated together with the fixing belt 301.
The fixing flange 300 has an internal diameter larger than an external diameter of the fixing belt 301, and the fixing belt 301 is pressed by a pressure roller 302 to cause a deformation, whereby an external peripheral surface of the fixing belt comes into contact with a internal receiving face 300a of the fixing flange 300 to generate a frictional force. Such frictional force causes the fixing flange 300 to rotate along the rotation of the fixing belt 301.
The fixing belt 301, being supported by the internal receiving face 300a, is prevented from being detached out of the fixing flange 300 even when the fixing belt 301 is moved in the longitudinal direction.
However, the fixing belt 301 in the course of its rotation is deformed by the pressure roller 302 in the vicinity of the fixing nip portion N, repeating motions in the radial direction (indicated by hatched portions a, b in FIG. 23) with respect to the fixing flange 300. Consequently, even though the fixing flange 300 rotates together with the fixing belt 301, a certain frictional contact is unavoidable between the edge portion of the fixing belt 301 and the fixing flange 300.
There is also proposed a configuration, as shown in FIG. 24 in a cross-sectional view seen perpendicularly to the longitudinal direction, in which a fixing flange 303 has a tapered shape toward an end thereof so as to form an angle θ equal to or larger than 90° between a longitudinal extension of the fixing belt 301 and a face of the fixing flange 303 in contact with the edge face of the fixing belt 301 thereby preventing a frictional contact on the edge face of the fixing belt 301 to a certain degree when the fixing belt 301 is deformed in the course of rotation thereof by the pressure roller 302, but a frictional contact in the radical direction between the edge face of the fixing belt 301 and the fixing flange 303 inevitably takes place in areas in the vicinity of the fixing nip portion N where the deformation starts and the deformation is finally restored.
Also in case the fixing belt 301 has a high biasing force, there may be generated a frictional force in a receiving portion of the fixing flange 303 and the fixing flange 303 may not rotate in synchronization with the fixing belt 301. In such situation, since the fixing flange 303 and the fixing belt 301 have different rotating speeds, a relatively strong force is applied to the fixing belt if the fixing flange 303 and the fixing belt 301 are not well mutually slidable. In such case, in case the fixing flange or the fixing belt is inclined to cause a contact therebetween in the vicinity of the nip, a very high buckling pressure is generated to eventually cause a fissure in the fixing belt.
Also in case the angle θ is selected large, the fixing flange 303 is drawn into a direction indicated by an arrow in FIG. 24 by the longitudinal biasing force of the fixing belt 301 to generate a strain in the fixing belt 301 in the longitudinal direction thereof, thereby increasing the stress thereon and resulting in a destruction thereof by fatigue.
Particularly a fixing belt 301 prepared by a plastic working such as stroking has a large retentive stress in the metal layer, and tends to cause a fissure starting from a small crack on the edge face of the fixing belt or a destruction by fatigue, by a frictional contact on the edge face or by a strain in the longitudinal direction.
In particular, a color image forming apparatus requires a higher pressure in the fixing nip portion N in comparison with a monochromatic image forming apparatus. This is because, since a color image is formed by superposing toners of plural colors and the height of the toner image becomes higher than in the monochromatic image forming apparatus, a higher pressure is required for forming a uniformly smooth image in the fixing portion.
However, a larger pressure in the fixing nip portion N increases the biasing force of the fixing belt in the longitudinal direction, thereby facilitating the fissure formation by the buckling of the fixing belt.